Alot of the older Gen 5 counters still work fine against Milotic even in Gen 6.
Like you said, Celebi works but older UU counters like Guts Heracross, Roserade with Natural Cure, Togekiss which has access to Heal Bell and Roost.
Bulky substitute Pokemon work too, with things like Shaymin and Abomasnow which can get up a substitute, and bash it with super effective attacks or sap it with leech seed.
Heliosk would work as well I would say, since Dry Skin allows it to absorb scald and it can bash Milotic with super-effective electric STAB.
Any physical Pokemon which preferably isn't weak to water with guts works fine too.
